Doses of the “most promising” potential vaccine against the Bundibugyo virus that is causing an Ebola outbreak in central Africa will not be available for six to nine months, the World Health Organization said on Wednesday, as the number of suspected cases rose to 600.

Dr Tedros Adhanom Ghebreyesus, the director-general of the WHO, told a press briefing on the outbreak in the Democratic Republic of the Congo and Uganda, that there had been 139 deaths, with numbers expected to rise.

Officials said they believe the disease may have started its spread “a couple of months ago”, aided by a “super-spreader event”, possibly a funeral, in early May.

The security situation in Ituri province, where more than 100,000 people have been displaced in recent months because of armed conflict, had complicated detection efforts, Tedros said. Health facilities could not provide care or surveillance for disease outbreaks if health workers were fleeing, he said.

Other illnesses endemic to the region, such as malaria and typhoid, have the same early symptoms as Ebola which can also delay diagnosis, he added.

Tedros said criticism of the organisation by US secretary of state Marco Rubio, who said the WHO had declared the outbreak “a little late”, was probably based on “a lack of understanding”.

“Maybe on what the secretary said, it could be from a lack of understanding of how IHR [international health regulations] works, and the responsibilities of WHO and other entities. We don’t replace the country’s work, we only support them,” said Tedros.

The Trump administration withdrew the US from the WHO earlier this year.

Dr Vasee Moorthy, who leads the WHO’s research and development blueprint, said the most promising potential vaccine against Bundibugyo uses the same basis as Ebola vaccines that target the Zaire strain.

“There are no doses of this which are currently available for clinical trial … The information that we have is this is likely to take six to nine months,” he said.

Doses of an alternative, which uses the same platform developed by Oxford University as AstraZeneca’s Covid jabs, could be available for clinical trials in two to three months, he said – but there is “a lot of uncertainty” as data from animal tests of efficacy are not yet available.

The outbreak was made public by African health officials on Friday, and the WHO declared it a public health emergency of international concern early on Sunday morning.

Tedros said: “WHO assesses the risk of the epidemic as high at the national and regional levels, and low at the global level.”

Modelling from Imperial College London suggests there could already be more than 1,000 cases of Ebola in the affected region.

Officials told the briefing that access issues, including frequently cancelled flights, were complicating efforts to get tests and other supplies to Ituri province.

“Our absolute priority now is to identify all the existing chains of transmission,” said Chikwe Ihekweazu, the WHO emergencies lead. “That will then enable us to really define the scale of the outbreak and be able to provide care.”